1. The Field of the Invention
The present invention relates to an office-line trunk circuit selectively operating on ground-start and loop-start modes, a method of controlling the circuit, and a control program for the circuit, and in particular, to (i) an office-line truck circuit serving both on the ground-start and loop-start modes, in which insulation between a primary office line and a secondary office line is secured by allowing a high-resistance detection circuit to detect the ground potential, which eliminates the need for using a de-linking circuit in a ground-start circuit in cases where a switching circuit and a loop-start mode are used, so that the detection is performed without a self power supply to be placed on the primary side of the office line, (ii) a method of controlling the office-line trunk circuit, and (iii) a control program for the office-line trunk circuit.
2. Related Art
In a conventional system in which both of a PBX and an office exchange are communicably connected to each other and the same hardware is used to selectively realize the function of loop start and ground start, it is necessary to have hardware including switch circuits.
There is another conventional system in which the switching is unnecessary but a self power supply is required. In this system, however, it is difficult to establish insulation between the primary side (i.e., an office exchange side) and the primary side (i.e., a PBX side) of an office line.
One conventional technique is disclosed by JP-A-S56(1981)-122552. This publication provides a subscriber's terminal circuit which is connected with a local telephone exchange and which has loop-start and ground-start modes. This subscriber's terminal circuit is characterized by the following configurations and operations. In the loop-start mode, 1) a diode bridge circuit equipped with a resistor, capacitor and photo coupler detects a ringing signal outputted from the local telephone exchange through both a tip line and a ring line, and 2) when the local subscriber's terminal circuit is brought into its service mode, both of a talk current supply resistor and a transistor in a DC amplifier are made to establish a connection between the tip line and the ring line. This connection allows a comparative circuit to make a comparison between a voltage obtained by dividing the voltage between the chip and ring lines and a voltage obtained by feed-backing, though a feedback resistor, a voltage between the talk current supply resistor and the transistor of the DC amplifier. Information resultant from the comparison is used to control a base current of the transistor. In addition, a circuit consisting of a shunt resistor and a loop current detection photo coupler, which are connected in series with each other, is connected in parallel to the talk current supply resistor to detect a loop current. There is also provided a circuit to protect the electronic circuits including the comparative circuit and the transistor from surge voltage. On the other hand, 3) in the ground-start mode, a start signal from the local telephone exchange is detected by both a high-resistance resistor and a photo coupler for detecting slight-amount current, which are connected in series between the chip and ring lines. In addition, when the start is made from the subscriber's terminal circuit, a tip line ground signal sent back from the local telephone exchange is detected by the photo coupler.
For example, in the loop-start mode, a switch is turned off, during which time the ringing signal from the local telephone exchange is rectified by a diode bridge, before being detected by a first photo coupler.
In the service mode, an audio signal is bypassed by a capacitor so that DC voltage is applied to the non-inverting (+) terminal of an operational amplifier. The output thereof is dedicated to control of base currents of transistors and a shunt current from a loop current though the chip and ring lines is detected by a second photo coupler.
Moreover, in the ground-start mode, the switch is turned off to cause a third photo coupler to detect the start signal from the local telephone exchange. Accordingly, when the subscriber's terminal circuit activates the local telephone exchange, a start completion signal is detected by a fourth photo coupler.
In cases where, under the ground-start mode, the subscriber's terminal circuit activates the local telephone exchange and the fourth photo coupler detects the start completion signal, the subscriber's terminal circuit detects the ground from the local telephone exchange by using a −48V power supply in the circuit itself for the confirmation of the start completion signal.
As described, the switch is operated to select either the loop-start mode or the ground-start mode, so that the −48V power supply in the subscriber's terminal circuit itself is used to confirm the start completion signal.
Accordingly, the use of the −48V power supply in the subscriber's terminal circuit makes it difficult to establish insulation between the primary side (i.e., an office exchange side) and the primary circuit (i.e., a PBX side) of the office line.
Another conventional technique is proposed by JP-A-S62(1987)-220095. This conventional publication discloses another type of office-line trunk circuit to detect the ground potential. The detection technique according to this publication eliminates the need for placing the de-linking circuit in the loop-start mode. This is because the ground detection is performed by making use of the potential of the self negative power supply only when the station is started under the ground-start mode.
When there is an incoming-call in this trunk circuit,-a US technique is performed such that a ring line remains as a negative potential with no changes in the potential and the tip line changes from its open state to the ground. Current, which is based on changes in the potential on the tip line, flows through a current monitoring circuit, and detected by a main detection element. A common control unit is notified of the incoming. When making the office start for the next outgoing, the ground potential is basically sent to the ring line. When the exchange-side circuitry in the office operates to send a signal indicating the ground to the tip line, changes in the potential to the ground are detected with current flowing from the current monitoring circuit to the negative power supply in the subscriber's terminal circuit.
Accordingly, in the case of this conventional detection technique, only when the office is activated on the ground-start mode, the potential of the self negative power supply is utilized for detecting the ground. Hence, if it is desired to apply this technique to establishment of the insulation between the primarily and secondary circuits of the office line, there is no alternative but to insulate the negative power supply itself from the secondary circuit.
In addition, another conventional technique is proposed by JP-A-H03(1991)-188793. Disclosed by this publication is a PBX capable of eliminating the need for setting a program when the exchange is installed. This elimination is realized through distinguishable registration of the type of a relevant contract office line, by performing ground-start type and loop-start type of dialing in response to detecting the power-on or reset of the PBX.
In this PBX, a power-on/reset detection unit detects an “on” state of the power or a reset state of the power, which then causes a controller to close a relay contact and to perform the ground-start type of dialing.
In this case, if the contract office line is the ground-start type of line, the controller uses a Tip ground detection circuit to identify the type and to register in a memory information showing that the identified office line is the ground-start type.
By contrast, when the contract office line is the loop-start type of line, the controller uses the Tip ground detection circuit to determine that the office line is not the ground-start type. The controller then moves to the loop-start type of dialing. In order to this dialing, the controller causes the relay contact to be open, both a Ring terminal and a Tip terminal to be coupled with a rectifying circuit. This controls a DC loop formation/loop current detection circuit identify that the contract office line is the loop-start type, and the identified information indicative of the loop-start type is registered in the memory. In order to detect the ground of the contract office line, the Tip ground detection circuit uses a −48V power supply placed in the PBX itself.
As described above, even in the conventional PBX, the −48V negative power supply in the exchange itself is used. Hence it is difficult to establish the insulation between the primary side (i.e., the contracted office side) and the secondary side (i.e., the exchange side) of the office line.
Another conventional technique is also proposed by JP-A-S59(1988)-168795.
This reference discloses a loop-start/ground-start line interface circuit, in which the operations are selected between the loop-start mode and the ground-start mode by performing a switch over responsively to a signal from either a loop-start circuit or a ground-start circuit.
In this conventional line interface circuit, a PBX is connected to an interface circuit via a telephone line circuit equipped with a tip conductor and a ring conductor. Via these conductors, line feed current is fed to a first differential amplifier and a second differential amplifier, respectively. A common-mode amplifier provides these differential amplifiers with mutually-equal reference signals, respectively. The common-mode circuit is switchable in such a manner that, under the loop-start mode, its connection is selective according to differences in the potential and, under the ground-start mode, the circuit is connected to a predetermined fixed bias voltage. In this PBX, detecting the ground (by a ground detector) is carried out using a negative power supply disposed in the PBX itself.
Accordingly, in the above conventional configuration, it is difficult to secure the insulation between the primary side (i.e., the contract office side) and the secondary side (i.e., the PBX side) of the office line, because the negative power supply, that is, −48V power supply, disposed in the PBX itself is used.
As described so far, if it is desired that the same office-line trunk circuit selectively performs both the loop-start operation and the ground-start operation, it is required for the circuit to have additional hardware means such as switch circuit. In contrast, when the circuit does not such a switch circuit, the circuit should use a self power supply disposed therein, resulting in that it is difficult to insulate the primary side (i.e., an office exchange side) from the secondary side (i.e., a PBX side) in an office line.